1. Field of the Invention
The present invention relates to video surveillance systems for monitoring areas during times of inactivity, for example: areas within banks, department stores, warehouses, or industrial facilities during hours when such facilities are closed. Such systems alert security personnel, and start video cassette recorders when, e.g., an intruder or a fire is detected.
2. Description of the Prior Art
In the prior art many security and intrusion detection systems based on non-video sensors, such as microwave sensors, geophones, magnetic seismic sensors, and ported coax cables, have been used to alert the appropriate authorities as to an unauthorized intrusion into an area. U.S. Pat. No. 4,651,143 issued to Yamanaka discloses a security system that uses sensors to detect intrusion. Once an intruder is detected a video recorder is activated, and a remote monitoring site is alerted.
U.S. Pat. No. 5,239,459 issued to Hunt et al. disclose a security system that requires several sensors in various zones within a structure to detect intrusion. Also, the Hunt et al. system uses adaptive learning networks to distinguish between an intrusion and a nuisance. Security systems of the type disclosed by Hunt et al. have the disadvantages of great cost and complexity, and of requiring sensors to be installed throughout a structure. For this reason there has been a trend in the art toward intrusion detection systems using video cameras. A single video camera can monitor a large area thus eliminating the need for large numbers of sensors within a structure. Further, video camera based systems can be installed at lower cost with little or no modification to the premises.
Various arrangements have been proposed for monitoring a scene using a video camera and activating various alarms and recording devices in response to any changes detected in the scene under surveillance. U.S. Pat. No. 3,825,676 issued to Ramsden, Jr. discloses a change detector circuit for detecting a change in the video signal from a video camera used as part of a surveillance system for monitoring an area to be protected. The circuit disclosed by Ramsden, Jr. comprises a signal integrator and a storage device. The video signal from the camera is sampled and the sampled signal is integrated using the integrator. The output of the integrator is temporarily stored in the storage device. The integrator is then reset to zero. The signal from the camera is again sampled and the sample integrated using the integrator. The output of the integrator is compared to the stored value, and an alarm is generated if any difference between the stored value and the most recent integrator output is detected. The value stored in the storage device is updated with the most recent integrator output and the integrator is reset to zero. This cycle is then continuously repeated for continuous operation of the change detector.
U.S. Pat. No. 3,828,125 issued to Fagan et al., discloses a surveillance system where the video signal from the camera is passed through various filters to reject portions of the video signal corresponding to large areas of uniform brightness and fine detail, such that only transitions from reasonably large objects are reflected in the filtered signal. The filtered signal is then fed to an average detector which produces an output voltage used to operate an indicator or alarm device. Fagan et al. also disclose the use of a sliding window amplifier to prevent false alarms due to slow changes in ambient lighting and rapid transient changes such as those caused by a bird flying through the monitored area. Fagan et al. also disclose the use of the alarm signal to activate a video recorder for obtaining a permanent record of the disturbances in the area under surveillance. Due to circuit noise, systems such as those disclosed in the Fagan et al. and Ramsden, Jr. patents inherently are not sensitive enough to reliably detect certain types of motion.
U.S. Pat. No. 3,932,703, issued to Bolsey, discloses a motion detector wherein the video signal is filtered to remove random high frequency noise and attenuate lower frequency portions of the video signal. The filtered signal is then fed to a flip-flop which changes the polarity of its output in response to each zero crossing in the filtered signal. The output of the flip-flop is fed to one input of a comparator gate, while a reference signal stored on a magnetic medium is fed to the other input of the comparator gate. The comparator gate output goes high if any mismatch exists between the flip-flop output and the reference signal. The output of the comparator gate is fed to an integrator which generates a greater than zero output which continuously increases as long as the comparator gate output is high. A threshold detector causes an alarm to be triggered when the output of the integrator passes a preset threshold.
The drawback of the Bolsey system is that changes in a single video frame can cause an alarm. Events in a single frame are generally too rapid to raise a security concern. In addition, because the Bolsey system relies on filtering to prevent false alarms, it is prone to false alarms under low light conditions when the signal to noise ratio is relatively low. To remedy this defect, Bolsey is forced to employ a complex scheme to prevent this type of false alarm. Bolsey uses an alarm disabling gate between the comparator and the integrator to prevent this type of false alarm. This gate is rendered conductive only if the filtered signal is above a certain threshold as determined by an additional threshold detector circuit. Thus the Bolsey system will be rendered totally inoperative when the ambient light falls below a threshold corresponding to the filtered signal threshold for rendering the alarm disabling gate conductive.
This feature also adds to the complexity of the operation of the Bolsey system, because the operator has to set the filtered signal threshold by empirically determining the level at which the filtered signal becomes masked by noise. Further, the Bolsey system prevents false alarms due to slow changes in ambient light conditions, for example, due to the setting of the sun, by making the amplitude of the filtered video signal independent of illumination. Thus the Bolsey system cannot distinguish between changes in ambient light, such as changes due to the setting of the sun, which do not require a security alert, and those, such as the blowing of a light bulb or the turning on of a light, which may require a security alert.
U.S. Pat. No. 3,988,533 issued to Mick et al. discloses a motion and intrusion detection system which digitizes the value of the video signal at discrete points within the visual field of the video camera. The values of the video signal at these same points in a subsequent frame are digitized and compared to the values from the previous frame by digitally subtracting one set of values from the other. In the system of Mick et al., the frames are compared at specific sample points. If a difference beyond a preset threshold is detected at a given sample point, then an alarm condition exists at that particular point. The system of Mick et al. keeps a count of sample points having an alarmed condition and a count of frames having an alarm condition. If a preset number of alarm conditions is received within a preset period of time, then an alarm signal is generated. The Mick et al. system has the disadvantage of requiring complex synchronization circuitry to ensure that the exact same points are sampled in each frame. Further, the Mick et al. system requires an extremely fast A/D converter which is expensive and requires sophisticated software for its operation.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.